来自Base Ctor的纯虚函数调用 [英] Pure Virtual Function call from Base Ctor

问题描述

请考虑以下示例代码：

  #include< iostream& 
 
 using namespace std; 
 
 class base 
 {
 public：
 base（）
 {
 bar（）; // Line1 
 this-> bar（）; // Line2 
 base * bptr = this; 
 bptr-> bar（）; // Line3 
（（base *）（this）） - > bar（）; // Line4 
} 
 
 virtual void bar（）= 0; 
}; 
 
 class derived：base 
 {
 public：
 void bar（）
 {
 cout< vfunc in derived class\\\
; 
} 
}; 
 
 int main（）
 {
 derived d; 
} 
  

上面的代码有纯虚函数 bar 在基类中在派生类中被覆盖。纯虚函数 bar（）在基类中没有定义。

现在关注 Line1 ， Line2 ， Line3 和 Line4 。

我理解： Line1 提供编译错误，

为什么 Line2 和 Line4 不提供编译错误与上面我理解语句中提到的相同的原因？ Line2 和 Line4 中的调用将最终导致链接器错误



1. 为什么 Line3 既不提供编译错误也不提供链接器错误，

纯粹的运行时异常

通过构造函数调用虚函数：

未定义行为

解决方案

/ strong>&编译器可以自由地显示任何行为。

参考：

C ++ 03 Standard 10.4 / 6：

可以从抽象类的构造函数（或析构函数）调用成员函数;直接或间接对虚拟函数进行虚拟调用（10.3）

C ++标准定义了未定义的行为：

[defns.undefined] 1.3.12未定义的行为

$ b $例如在使用错误的程序结构或错误的数据时可能出现的行为，本国际标准对此没有要求。当本国际标准省略对行为的任何明确定义的描述时，也可以预期未定义的行为。 [注意：允许的未定义行为的范围包括完全忽略具有不可预测的结果的情况，在以文件化的环境特征（有或没有发出诊断消息）的翻译或程序执行期间行为，终止翻译或执行（随着诊断消息的发出）。许多错误的程序结构不会产生未定义的行为;他们需要诊断。]

Consider the following sample code:

#include <iostream>

using namespace std;

class base
{
   public:
      base()
      {
         bar(); //Line1
         this->bar(); //Line2
         base *bptr = this; 
         bptr->bar(); //Line3
         ((base*)(this))->bar(); //Line4
      }

      virtual void bar() = 0;
};

class derived: base
{
   public:
      void bar()
      {
         cout << "vfunc in derived class\n";
      }
};

int main()
{
   derived d;
}

The above code has pure virtual function bar() in base class which is overriden in the derived class. The pure virtual function bar() has no definition in base class.

Now focus on Line1, Line2, Line3 and Line4.

I understand : Line1 gives compilation error, because pure virtual function cannot be called from ctor.

Questions:

1. Why does Line2 and Line4 give no compilation error for the same reason mentioned in I understand statement above?. The calls in Line2 and Line4 will eventually cause linker-error only.

2. Why does Line3 give neither compilation error nor linker error but gives run-time exception only ?

Real-Life example of UB when Pure virtual function call through constructor:

解决方案

Calling an Pure virtual function from constructor is an Undefined Behavior & the compiler is free to show any behavior.

Reference:
C++03 Standard 10.4/6:

"Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call (10.3) to a pure virtual function directly or indirectly for the object being created (or destroyed) from such a constructor (or destructor) is undefined."

The C++ standard defines Undefined behavior in:

[defns.undefined] 1.3.12 undefined behavior

behavior, such as might arise upon use of an erroneous program construct or erroneous data, for which this International Standard imposes no requirements. Undefined behavior may also be expected when this International Standard omits the description of any explicit definition of behavior. [Note: permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed. ]

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